Transistors used for most flat panel displays typified by a liquid crystal display device and a light-emitting display device are formed using silicon semiconductors such as amorphous silicon, single crystal silicon, and polycrystalline silicon provided over glass substrates. Further, transistors formed using such silicon semiconductors are used in integrated circuits (ICs) and the like.
In recent years, attention has been drawn to a technique in which, instead of a silicon semiconductor, a metal oxide exhibiting semiconductor characteristics is used in transistors. Note that in this specification and the like, a metal oxide exhibiting semiconductor characteristics is referred to as an oxide semiconductor.
For example, such a technique is disclosed that a transistor is manufactured using zinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductor and the transistor is used as a switching element or the like in a pixel of a display device (see Patent Documents 1 and 2).
Non-Patent Document 1 reports that defect states with extremely high densities of 1×1020/cm3 or more are observed in an amorphous In—Ga—Zn—O film and that the defect states are reduced in half by heat treatment.